"We are monitoring the situation," Shah said at the daily briefing. "I don't want to get ahead of anything that may or may not be announced on that front, but the Assad regime and Russia's actions on this front are on notice."

Airstrikes killed 28 civilians Monday and after a suspected chlorine attack the day before.

The bombings intensified after Syrian insurgents downed a Russian Su-25 jet over the weekend, the first time they scored such a major hit against the Kremlin.

Russia has waged a brutal aerial campaign against Syria's armed opposition since intervening on Assad's behalf in 2015.

Cease-fire deals have failed to quell the violence or restore humanitarian aid to besieged Ghouta, where 400,000 residents are holed up amid warnings of a looming humanitarian disaster.

But Shah declined to say whether the new round of bombing constituted war crime by Assad.

"Bashar al-Assad has already committed war crimes — and has gassed people with sarin," he told reporters. "He has already committed unthinkable acts and has done so with Russian support.